1. Field of the Invention
The present invention generally relates to a light scanning arrangement for and method of repetitively scanning targets, for example, optical codes, and, more particularly, to operating such a scanning arrangement under low electrical power conditions to minimize usage of electrical power and at high scan rates.
2. Description of the Related Art
Various optical code readers have been developed heretofore to read optical codes, such as bar code symbols applied to objects in order to identify each object by optically reading the symbol thereon. The bar code symbol itself is a coded pattern comprised of a series of bars of various widths and spaced apart from one another to bound spaces of various widths, the bars and spaces having different light reflecting properties. The readers electro-optically decoded the coded patterns to multiple digit representations descriptive of the objects. Readers of this general type have been disclosed, for example, in U.S. Pat. No. 4,251,798.
As disclosed in the known art, a particularly advantageous embodiment of such a reader resided, inter alia, in emitting a light beam, preferably a laser beam, emitted from a light source, preferably a gas laser or a laser diode, and in directing the laser beam to a symbol to be read. En route to the symbol, the laser beam was directed to, and reflected off, a light reflector of a scanning component. The scanning component moved the reflector in a cyclical fashion and caused the laser beam to repetitively scan the symbol. The symbol reflected the laser beam incident thereon. A portion of the incident light reflected off the symbol was collected and detected by a detector component, e.g., a photodiode, of the reader. The photodiode had a field of view, and the detected light over the field of view was decoded by electrical decode circuitry into data descriptive of the symbol for subsequent processing. The cyclically movable reflector swept the laser beam across the symbol and/or swept the field of view during scanning at a scan rate.
Several different types of scanning components were known in the art. For example, U.S. Pat. No. 4,251,798 disclosed a polygonally-shaped multi-faced wheel having mirrored outer surfaces on each of its faces. The wheel was mounted for rotation so that each mirrored outer surface was, in turn, positioned in an optical path along which the incident laser beam was directed to the symbol to be scanned. Other scanning components disclosed in U.S. Pat. No. 4,251,798 were a bimorph or ferroelectric-type oscillating element, as well as a speaker-type oscillating element, each element having an oscillatable mirror.
U.S. Pat. No. 4,387,297 and U.S. Pat. No. 4,496,831 disclosed a scanning component including an electric motor operative for reciprocatingly oscillating a reflector in opposite circumferential directions relative to an output shaft of the motor. U.S. Pat. No. 4,387,297 also disclosed a penta-bimorph scanning component. U.S. Pat. No. 5,099,110 disclosed a power saving scanning component wherein a scan mirror was oscillated by releasing stored energy in a hinge.
The light reflector need not have been a single planar mirror, but, as described in U.S. Pat. No. 4,760,248, could have been an optical component having a construction which was part concave and part planar. The reflector could be driven by an electrical motor in alternate circumferential directions along arc lengths less than 360° as described in U.S. Pat. No. 4,496,831, or, in a variant construction, could be rotated completely around an axis as described in U.S. Pat. No. 4,816,661.
In still another variant construction, as described in U.S. Pat. No. 5,144,120, rather than using a reflector to effect the scanning action, the reflector was eliminated altogether in a so-called “mirrorless” design in which one or more of the other reader components were jointly or separately moved relative to one another to effect a reciprocating scanning action driven by an electrical motor. The other reader components included a light source component, e.g., a laser diode, and/or optical components, e.g., a focusing lens, and/or the photodetector component, e.g., a photodiode.
In all of the above-described scanning components, the scan rate was relatively low, on the order of less than 100 Hz. To increase the scan rate to a range from 350 Hz to 1200 Hz, U.S. Pat. No. 5,412,198 disclosed a resonance asymmetric scan element in which a scan mirror was oscillated by means of magnetic field interaction between a permanent magnet and an electromagnetic coil.
It was known in the art to mount the permanent magnet or the electromagnetic coil directly on the scan mirror to form a magnetically-driven oscillatable mirror assembly. It was also known to mount electrodes directly on the scan mirror in an electrostatically-driven oscillatable mirror assembly. This, however, increased the mass of the oscillatable assembly, thereby requiring more electrical power to drive the assembly and limiting the scan rate. In readers of the hand-held type, wherein electrical power is supplied by on-board batteries, a high electrical power requirement reduces the working lifetime of the batteries. Also, there are many applications, not limited to reading bar code symbols, where a high scan rate in excess of 1200 Hz and, indeed, at inaudible frequencies greater than 20 kHz is desired.